Introduction

This note defines freezing time and explains why it is
important, what it depends on and how it can be measured. Estimated freezing
times are often inaccurate, and the note emphasizes the importance of measuring
freezing time during the process. Advisory Note 20 gives advice on measuring
fish temperature, and Note 94 outlines the principles of freezing fish; these
should be read in conjunction with this note.

A list of measured freezing times for a number of fish
products is given at the end of the note.

What is freezing time?

Freezing time is defined here as the time taken for the
temperature of the warmest part of the fish, usually the centre, to be reduced
to -20°C.

The recommended storage temperature for frozen fish in the UK
is -30°C and, to ensure that the fish is frozen quickly, the temperature of
the freezer must be lower than this. Thus when the surface of the fish is at
freezer temperature and the warmest part is down to -20°C, the average
temperature of the fish on removal from the freezer will be close to the
required storage temperature of -30°C.

How does fish freeze?

Fish muscle usually contains 60-80 per cent water, depending
on species and season, but the tissue fluid contains salts and other compounds
in solution and hence the muscle freezes in rather a different manner from
water. The change in heat content of white fish muscle and of water when the
temperature changes is shown in figure 1; it can be seen that fish has to be
reduced to a much lower temperature than water before most of the heat is
removed and the fish is completely frozen.

Fig 1

Why Measure Freezing
Time?

The size of a freezer and the capacity of its refrigeration
plant depend on the quantity of fish to be frozen and the freezing time of the
particular product; the plant designer must be given this information. The
longer the freezing time, the bigger the freezer has to be for a given output.
For example an output of 1 tonne an hour of a product that takes 1 hour to
freeze will require a freezer to hold 1 tonne but if the freezing time is 2
hours the freezer has to hold 2 tonnes. Where several products are to be frozen,
the freezer should be designed to accommodate the required load of the product
with the longest freezing time; the freezing rate for that freezer is then
fixed.

A knowledge of product freezing times will help especially to
reduce errors in operating air blast and immersion freezers which, because of
their design, are more versatile in the shape and size of product they can
accommodate and can therefore be overloaded. A more detailed discussion of the
problems of loading blast freezers is given in Advisory Note 35.

The type of freezer will greatly influence the freezing time;
for example a product will normally freeze faster in an immersion freezer than
in an air blast freezer operating at the same temperature.

Operating temperature

The colder the freezer the faster the fish will freeze, but
the cost of freezing increases when the freezer temperature is reduced, and in
practice freezers are usually designed to operate a few degrees below the
required storage temperature of the product. For example, plate freezers usually
operate at about -40°C and blast freezers at about -35°C to freeze
products for storage at -30°C.

Air speed in a blast freezer

The effect of air speed on freezing time is shown in figure
2.

Freezing time is reduced as air speed is increased; up to
about 5 m/s the reduction is considerable, but at higher speeds there is little
further improvement.

Product temperature before freezing

The warmer the product, the longer it will take to freeze; the
initial temperature of the product should be given when quoting a freezing time.
Fish should be kept chilled before freezing both to maintain quality and to
reduce freezing time.

Fig. 2

Product thickness

The thicker the product the longer the freezing time. For
products less than 50 mm thick, doubling the thickness may more than double the
freezing time; doubling a thickness of 100 mm or more may increase the freezing
time fourfold.

Product shape

In a freezer that is suitable for single fish, for example a
blast freezer, a round fish will freeze in about two thirds of the time taken
for a flat fish of the same thickness; thus product shape may be
important.

Product contact area and density

In a plate freezer, poor contact between product and plate
results in increased freezing time. Poor contact may be due to ice on the
plates, packs of unequal thickness, partially filled packs or voids at the
surface of the block. Surface voids are often accompanied by internal voids;
these cause poor conduction of heat, thus increasing freezing time and also
reducing the density of the block. The relationship between freezing time, block
density and contact area for 100 mm blocks of white fish is shown
below.

Block densitykg/m3

contact area%

freezing timeh

800

48

3.0

780

45

3.0

650

29

3.8

650

21

4.0

Product packaging

The type of wrapping material has some influence on freezing
time, but often the method of wrapping, particularly when air is trapped between
wrapper and product, has even more effect. The following example illustrates the
point. Kippers in a wooden box with the lid on take 15 hours to freeze in an air
blast freezer. Kippers in an aluminium box of the same shape and size and with
the lid on take 12 hours, but if the lid is taken off the wooden box the
freezing time is only 8 hours, because there is no trapped air.

Species of fish

The lower the water content of a fish, the smaller the amount
of heat to be extracted. Since water content goes down as fat content goes up,
fatty fish like herring for example may freeze more quickly than lean fish. But
since fat content varies with season, it is usually safer to assume the heat
content figure for white fish muscle, that is about 320 kJ/kg as shown in figure
1, in any calculations.

How is freezing time
measured?

Freezing time is best measured by inserting a special type of
thermometer known as a thermocouple into the product so that it measures the
temperature of the last part to fall to -20°C. The correct use of
thermocouples in fish is described in Advisory Note 20. Since a single
thermocouple slightly out of position can give a misleading answer, as shown in
figure 3, it is advisable to insert two or three thermocouples in each sample to
ensure success. Samples should be measured at several positions in the freezer
to check the uniformity of freezing. The leads from the thermocouple to the
instrument should be led out of a sample along the centre line to reduce heat
conduction, and should be anchored as shown in figure 4.

Fig. 3. POSITION OF THERMOMETER IN
FISH

It is impossible to tell whether a product is completely
frozen merely by examining the surface; probing the surface with a screwdriver
or a knife is useless as an indication of whether the centre has reached the
required temperature.

Fig. 4. CORRECT METHOD OF LOCATING
THERMOCOUPLES FOR FREEZING

Can freezing times be
calculated?

Freezing times can be calculated, but there is usually
insufficient information to make an accurate estimate. Freezing times can be
predicted fairly accurately for uniformly shaped products like laminated blocks
under ideal conditions, but variations in freezer temperature, the presence of
wrappers, and other factors can still make the calculation complicated and
unreliable. Freezing time should always be measured during the process; once
established in this way, it will alter only if the product or the operating
conditions are changed.

How accurate must a freezing time
be?

A small error in a short freezing time may be much more
significant than the same error in a long freezing time. For example an error of
5 minutes in measuring a freezing time of 8 hours is unlikely to have any effect
on design or operation, but an error of 5 minutes in measuring a freezing time
of 20 minutes could mean that for example the cost of an expensive stainless
steel belt in a continuous shrimp freezer would be 25 per cent higher than
necessary.

Specifying freezing
time

Processors often ask for the freezing time of a fish product
without giving any other information; such a question is impossible to answer
precisely. For example the answer for shrimp might be anything from 5 to 50
minutes; 5 minutes might be correct for individual meats frozen in liquid
nitrogen spray at -80°C, while 50 minutes might apply to a 25 mm thick
carton in a plate freezer at -35°C. Clearly it is necessary to specify both
the product and the operating conditions when quoting a freezing time.

Freezing times for fish
products

The following are observed times for a number of fish
products; although the freezing time of a new product in a particular freezer
should always be measured in the recommended manner, these typical freezing
times will give designers and operators some idea of what to expect in
practice.